This invention relates in general to the formation of coatings by plasma spraying techniques, and, more particularly, to the production of magnetic-cement dielectric coatings by plasma spraying.
In the past, magnetic-cermet dielectric coatings have been formed by coating a mixture of metal particles and ceramic particles in a liquid carrier onto a substrate, evaporating the liquid and heating the assembly to the sintering temperature of the ceramic. A dielectric layer results, with the metal particles sufficiently separated (by spacing or by in-situ formation of insulating oxide layers) so that the coating has sufficiently low electrical conductivity.
While suitable for some applications, this process has a number of disadvantages and problems. Parts to be coated are limited in size to the sizes of sintering furnaces available. Since the sintering temperature is quite high, often over 800 degrees C., depending on the ceramic glass frit employed, substrates such as polymeric materials, or carbon-carbon composites that would be damaged at the sintering temperature in air cannot be used. Also, cooling from the sintering temperature may, where the coating and substrate have significant differences in coefficient of thermal expansion (CTE), cause cracking or delamination in the coating. Repairs of damaged areas are particularly difficult where the entire part must be exposed to the sintering conditions.
Attempts have been made to form magnetic-cermet dielectric coatings by plasma spraying, with little success since the coatings prepared are excessively conductive even with a metal content as small as about 10 vol %.
The ability to coat a variety of substrates of various sizes would expand the usefulness of magnetic-cermet dielectric coatings beyond the present microwave absorption uses. For example, microwave absorbing coatings on cookware and browning dishes for use in microwave ovens would benefit from the capability of induction heating at or near food surfaces. These coatings would have applications in induction heating coatings for industrial processes such as heat treatment of non-ferrous alloys. They also may find application in interference fit assembling, welding, brazing, etc. and in the production of magnetic coatings on non-ferrous alloy plates for computer memory disk drives. Because of their high temperature resistance, they will have applications as a microwave absorber for aircraft engine exhaust components and the like.
Thus, there is a continuing need for improved methods of forming magnetic-cermet dielectric coatings having improved durability, high temperature resistance and microwave absorption characteristics.